The method and apparatus of the present invention are improvements over the autocirculation method and apparatus disclosed in my prior U.S. Pat. No. 4,238,462 and is useful for gas-liquid mass transfer where a liquid is contacted with two different gases in separate contact zones. In a catalytic process for removing hydrogen sulfide gas (H.sub.2 S) from a fluid stream using a ferric iron catalyst, such as a ferric iron chelate, the ferric iron is reduced to the ferrous state when reacted with hydrogen sulfide. The series of reactions involved in catalytically oxidizing hydrogen sulfide gas to form elemental sulfur using a ferric iron chelate catalytic reagent can be represented by the following equations, as set forth in my prior U.S. Pat. No. 4,238,462: EQU H.sub.2 S(Gas)+H.sub.2 O(Liquid).revreaction.H.sub.2 S(Aqueous)+H.sub.2 O(Liquid) EQU H.sub.2 S(Aqueous).revreaction.H.sup.+ +HS.sup.- EQU HS.sup.- .revreaction.H.sup.+ +S.sup.= EQU S.sup.= +2(Fe*Chelate).sup.+3 .fwdarw.S.degree. (Solid)+(Fe*Chelate).sup.+2
By combining these equations, the resulting equation is: EQU H.sub.2 S(gas)+2(Fe*Chelate).sup.+3 .fwdarw.2H.sup.+ +2(Fe*Chelate).sup.+2 +S.degree.
In order to have an economical, workable process to effect catalytic oxidation of the hydrogen sulfide, it is essential that the hydrogen sulfide gas be brought continuously into intimate contact with the chelated iron solution and that the ferrous iron chelate formed in the above described manner be continuously regenerated by oxidizing to ferric iron chelate by intimate contact with dissolved oxygen, preferably derived from ambient air. The series of reactions that take place when regenerating the required ferric iron chelate can be represented by the following equations: EQU O.sub.2 (Gas)+2H.sub.2 O.revreaction.O.sub.2 (Aqueous)+2H.sub.2 O EQU O.sub.2 (Aqueous)+2H.sub.2 O+4(Fe*Chelate)+2 .fwdarw.4 (OH.sup.-)+4 (Fe*Chelate).sup.3
By combining these equations, the resulting equation is: EQU 1/2O.sub.2 (Gas)+H.sub.2 O+2(Fe*Chelate).sup.+2 .fwdarw.2(OH).sup.- +2(Fe*Chelate).sup.+3
It will be evident from the foregoing equations that theoretically two moles of ferric iron must be supplied to the reaction zone in which the hydrogen sulfide gas is oxidized to form elemental sulfur for each mole of hydrogen sulfide gas treated, and in actual practice considerably more than the theoretical amount of iron is used. In a continuous process of removing hydrogen sulfide by contact with a catalytic ferric iron solution, the catalytic solution is circulated continuously between an absorber zone, where the H.sub.2 S is absorbed by the catalytic ferric iron chelate solution, and the solution reduced to ferrous iron in an oxidizer zone where the reduced ferrous iron is oxidized back to the ferric iron state. In order to avoid using high concentrations of iron in the catalytic solution, the rate of circulation should be high.
In the past, vanadium redox catalysts could not be used in an autocirculation process and apparatus, such as disclosed in my prior U.S. Pat. No. 4,238,462 because the vanadium redox reactions require more time to complete the H.sub.2 S oxidation reaction in the absence of dissolved oxygen before the solution exiting the absorber is exposed to oxygen.
The economics and workability of the Stretford process have depended upon a large volume of the metal vanadate redox solution. The reduced metal vanadate, after absorption of the H.sub.2 S (as HS.sup.- or S.sup.=) to form the metal vanadate in the +4 valance state is continuously regenerated to the +5 valance state by contact with dissolved oxygen for continuous use of the oxidized metal vanadate in the absorption zone to remove additional H.sub.2 S as elemental sulfur. The Stretford process chemistry is typically summarized according to the following steps:
Absorption and dissociation of H.sub.2 S into alkali: EQU 2H.sub.2 S(g)+2Na.sub.2 CO.sub.3 .fwdarw.2NaHs+2NaHCO.sub.3 ;
Bisulfide oxidation with metavanadate to form elemental sulfur and reduced vanadium: EQU 2NaHS+4NaVO.sub.3 +H.sub.2 O.fwdarw.Na.sub.2 V.sub.4 O.sub.9 +4NaOH+2S;
and
Vanadium reoxidation by dissolved molecular oxygen in the presence of ADA: ##STR1##
Prior to the autocirculation method and apparatus disclosed in my U.S. Pat. No. 4,238,462, the catalytic oxidation-reduction reactions for continuously removing hydrogen sulfide, or the like, from a fluid stream were carried out concurrently in the same reaction vessel by means of a process which can be referred to as an aerobic operation, or by means of a process in which the oxidation and reduction steps were carried out in separate reaction vessels in what can be referred to as an anaerobic operation (see U.S. Pat. No. 3,897,219). While an anaerobic operation may have certain advantages over an aerobic operation for treating some gas streams which must be recovered after H.sub.2 S removal, there is the extra expense involved in providing additional equipment, and the continuous pumping of large volumes of liquid from one vessel to the other increases operating costs.
The method and apparatus described in my U.S. Pat. No. 4,238,462 has been commercially successful, but the commercial use of that method and apparatus suffers from several disadvantages including some lack of control of residence time for gas-liquid contact in each of the reaction zones; no provision for liquid flow control; a relatively high thiosulfate production rate--4 to 6 percent by weight of the sulfur being converted to thiosulfate; and relatively high iron losses when the method and apparatus are used in the preferred embodiment for removing hydrogen sulfide from a gas stream.
Further, one skilled in the art will see that the autocirculation process as practiced commercially U.S. Pat. No. 4,238,462 comprises a well-stirred oxidation section, in which the composition of the bulk of the solution in the oxidizer is of necessity substantially the same as that circulating into the absorption zone. That is, it is nearly completely oxidized, so has a relatively high oxygen partial pressure and presents a minimum driving force for dissolution of oxygen in the solution.
In accordance with the process and apparatus of the present invention, the quantity of oxygen dispersed in a last oxidation stage can be controlled to prevent a substantial excess of dissolved oxygen in the polyvalent metal redox solution entering the absorption stage of the process so that sulfate, thiosulfate and other salts will be formed in the absorption zone of the process to a much lesser extent, enabling the process to be carried out at a more efficient, higher pH with little or no need for periodic addition of alkali to the polyvalent metal redox solution.
Finally, the autocirculation apparatus as presently practiced is not applicable to the Stretford process and such redox processes which rely on relatively slow reactions between the oxidized metal redox catalyst and dissolved HS.sup.- or S.sup.= ions because there is no effective residence time for the solution leaving the absorber before it enters the oxidizer. The absence of sufficient residence time contributes significantly to the formation of thiosulfate and sulfate salts by permitting the residual HS.sup.- and S.sup.= ions to enter the oxygen-rich oxidation zone where the non-specific side reactions between these species and dissolved oxygen can take place. The present method and apparatus overcomes this limitation, and makes possible full use of the autocirculation principle for Stretford and any other H.sub.2 S removal process that uses a catalytic redox polyvalent metal solution that is reduced and oxidized for converting H.sub.2 S to elemental sulfur, because the present invention provides for the insertion of a residence chamber through which the liquid exiting the absorber passes before entering the oxidizer. In the case of the LO-CAT.RTM. process, a residence time of 10 to 60 seconds may be adequate, whereas in the case of the Stretford chemistry 10 to 60 minutes may be required.